I think it's quite noticeable and hilarious how much hidden resentment of vegans and vegetarians come out in this thread. Obvs lots of people find it a bit weird or even annoying. I think that it's anyone's right to live as they choose, and as a host, or accompanying a vegetarian/vegan eating out, there needs have never bothered me or caused any trouble. Most vegans and vegetarians are quite flexible and accommodating about their needs, and will find something to make do in most situations. If they're not, and are demanding and moaning, then this is a personality feature that comes out in this way too, and not caused by their vegan/vegetarianism!
With regards to the original OP, then I hardly think that her reading out comments and bitching about the vegan guest can come as a surprise. Her resentment of the guest was clear from the first post. It sounded as if she didn't genuinely like the guest, and was prone to getting irritated at a small thing the guest would do. This may, or may not, have other legitimate reasons, perhaps not even to do with the guest's veganism.
On a general note. I find unwelcoming hosts unbearable. It is something I've seen several times: People invite you to their house, or someplace else, but in some cases they don't genuinely want to, they just feel pressured to throw a party or invite you. This underlying resentment then shines through in them not being genuinely hospitable, but instead being inflexible in meeting guests needs and unnecessarily anal about their behaviour, to the extent of even policing guests behaviour, like the OP did. I feel that if you don't genuinely want to throw a dinner party, or don't genuinely want a certain guest there, then don't! It's just uncomfortable for you and the guests. If you have to invite a certain person along who you don't want, then at least resolve to have good manners and be welcoming to that person. On the contrary, there's nothing better than a genuinely hospitable host, who go out of their way to make guests feel comfortable and relaxed. And this includes catering to guests different needs, and not being annoyed if they behave in small ways that you did not expect. Of course, if someone is genuinely rude or out of order, it is acceptable to feel pissed off about it, but small things like ASKING FOR A BITER OF FISH, really does not classify as rudeness.